Musnad Ahmad 27:977

'Affan narrated to us, 'Abd al-'Aziz ibn Muslim narrated to us, 'Abdullah ibn Dinar narrated to us, from 'Abdullah ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever buys food, let him not sell it until he has taken possession of it."